Pearlitic steel rail having excellent wear resistance and method of producing the same

ABSTRACT

This invention is directed to improve a wear resistance and a damage resistance required for a rail of a sharply curved zone of a heavy load railway, comprising more than 0.85 to 1.20% of C, 0.10 to 1.00% of Si, 0.40 to 1.50% of Mn and if necessary, at least one member selected from the group consisting of Cr, Mo, V, Nb, Co and B, and retaining high temperature of hot rolling or a steel rail heated to a high temperature for the purpose of heat-treatment, the present invention provides a pearlitic steel rail having a good wear resistance and a good damage resistance, and a method of producing the same, wherein a head portion of the steel rail is acceleratedly cooled at a rate of 1° to 10° C./sec from an austenite zone temperature to a cooling stop temperature of 700° to 500° C. so that the hardness of the head portion is at least Hv 320 within the range of a 20 mm depth.

The present application is a continuation reissue application of U.S.application Ser. No. 12/474,137 filed May 28, 2009 RE42,360, and U.S.application Ser. No. 12/474,137 is a continuation reissue application ofU.S. application Ser. No. 11/561,654 filed Nov. 20, 2006, now RE41,033,and U.S. application Ser. No. 11/561,654 is a continuation reissueapplication of U.S. application Ser. No. 10/974,048, filed Oct. 26,2004, now RE40,263 which is a reissue application of U.S. Pat. No.5,762,723, which issued on Jun. 9, 1998. The entire disclosures of theabove-described continuation reissue applications, the reissue patentand the patent are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This invention relates to a pearlitic steel rail which improves the wearresistance and breakage resistance that are required for rails at curvedzones of heavy load railways, and drastically improves the service lifeof the rails, and a method of producing such rails.

BACKGROUND ART

Attempts have been made to improve a train speed and loading as one ofthe means for accomplishing higher efficiency of railway transportation.Such an improvement in efficiency of railway transportation means severeuse of the rails, and a further improvement in the rail materials hasbecome necessary. More concretely, wear drastically increases in therails laid down in a curved zone of a heavy load railway and produces aproblem from the aspect of longer service life of the rails.

However, high strength (high hardness) rails using eutectoid carbonsteels and exhibiting a fine pearlite structure have been developed dueto the recent improvements in high-strength rail heat-treatmenttechnology as described below, and rail life in the curved zones in theheavy load railway has been remarkably improved.

(1) Heat-treated rails for ultra-heavy load having a sorbite structureof a fine pearlite structure at the head portion thereof (JapaneseExamined Patent Publication (Kokoku) No. 54-25490);

(2) Production method for low alloy heat-treated rails which improvesnot only the wear resistance but also the drop of hardness at a weldportion by adding an alloy such as Cr, Nb, etc. (Japanese ExaminedPatent Publication (Kokoku) No. 59-19173); and

(3) Production method for a high strength rail of at least 130 kgf/mm²produced by conducting accelerated cooling between 850° C. to 500° C. ata rate of 1° to 4° C./sec after rolling is completed or from a re-heatedaustenite zone temperature.

The characterizing feature of these rails is that they are high strength(high hardness) rails exhibiting the fine pearlite structure ofeutectoid carbon-containing steel, and the rails are directed to improvethe wear resistance.

In recent heavy load railways, however, an improvement in an axial loadof cargos (the increase of train loading) has been strongly promoted soas to further improve railway transportation efficiency. In the casewhere the rails are sharply curved, the wear resistance cannot besecured even when the rails developed as described above are used, andthe drop of rail life due to the wear has become a serious problem. Withsuch a background, the development of rails having a higher wearresistance than that of the existing eutectoid carbon steels has beenrequired.

The contact state between the wheel and the rail is complicated.Particularly, the contact state of the wheels is very different at theinner track rail compared to the outer track rail of the curved zone. Onthe outer track rail of the sharply curved zone of the heavy loadrailway, for example, the wheel flange is strongly pushed to the gagecorner portion by the centrifugal force and receives sliding contact. Onthe head top portion of the inner track rail of the curved zone, on theother hand, the rail receives great slipping contact having largecontact surface pressure from the wheel. As a result, in the case of thehigh strength wear-resistant rails according to the prior art whereinthe head surface hardness is uniform inside the cross-section of therail head portion, wear is promoted far more at the gage corner portionwhich receives the sliding contact of the outer track rail than the headtop portion which receives the slipping contact of the inner track rail.On the other hand, the progress of the wear is always slower at the headtop portion of the inner track rail than at the gage corner portion, andthe contact surface pressure from the wheel is always maximal.Therefore, fatigue damage builds up on the head top surface before it isworn out.

The contact state with the wheels tends to the state described above inthe high strength wear-resistant rails having uniform wearcharacteristics at the rail head portion according to the prior art,particularly on the inner track rail of the curve zone. Therefore, iffitting of the rail to the wheel is not quick at the initial wear stateimmediately after the laying of the rail, a local and excessive contactsurface pressure consecutively acts on the rail and surface damage dueto fatigue is likely to occur. In addition, even after fitting isestablished between the rail and the wheel, a large contact surfacepressure always acts on the head top portion and consequently, surfacedamage similar to so-called “head check”, which generally occurs at thegage corner portion, develops with plastic deformation because the wearis less.

To cope with this problem, there is a method which cuts off the surfacelayer of the rail head top portion before the rolling fatigue layer isbuilt up. Because the cutting work requires a long time and isexpensive, the following rail has been developed.

(4) A high strength and damage-resistant rail exhibiting the finepearlite structure of eutectoid carbon-containing steel wherein adifference of hardness is provided so that the hardness of the gagecorner portion is higher than that of the top head portion in thesectional hardness distribution of the rail head portion, in order tosecure the wear resistance equal to that of the conventional highstrength wear-resistant rails having a uniform head surface hardness inthe cross-section at the gage corner portion, and to reduce the maximumsurface pressure (to increase the contact area) by reducing the hardnessat the head portion and to improve the surface damage resistance due tothe wear promotion action (Japanese Unexamined Patent Publication(Kokai) No. 6-17193).

However, higher axial load of cargos (increase of railway loading) hasbeen vigorously promoted in recent years so as to attain higherefficiency of railway transportation, and even when the rails developedas described above are used, sufficient wear resistance cannot besecured at the gage corner portions of the outer track rail even thoughthey can prevent the surface damage by the periodically grinding of thehead top portion in the inner track rail at the sharply curved zone, andthe drop of rail life due to wear has been a serious problem.

DISCLOSURE OF THE INVENTION

The pearlite structure of the eutectoid carbon component, that has beenused in the past as the rail steel, has a lameller structure comprisinga ferrite layer having a low hardness and a tabular hard cementitelayer. As a result of observation of the wear mechanism of the pearlitestructure, the inventors of the present invention have confirmed thatthe soft ferrite structure is first squeezed out due to repetitivepassage of the wheels, and only hard cementite is then built upimmediately below the rolling surface, and work hardening adds to theformer, thereby securing wear resistance.

Therefore, the present inventors have found out through a series ofexperiments that the wear resistance can be drastically improved byincreasing the hardness of the pearlite structure to obtain a higherwear resistance, increasing at the same time the carbon content so as toincrease the ratio of the hard tabular cementite layer and thusincreasing the cementite density immediately below the rolling surface.

Further, the inventors of the present invention have paid specificattention to the increase in the carbon content which directly affectsthe improvement of the wear resistance, and have developed aheat-treatment method for stably obtaining a pearlite structure in thehypereutectoid steel. FIG. 1 is a diagram showing the results ofcomparison of the wear resistance between the eutectoid steel and thehypereutectoid steel on an experimental basis. The present inventorshave found out that the wear resistance can be drastically improved inthe hypereutectoid steel by an increase in the carbon content at thesame hardness (strength). The noteworthy point of this heat-treatmentmethod resides in that when the carbon content is increased, thepearlitic transformation nose (start) moves towards the short time areamuch more than in the eutectoid steel component materials and thepearlite transformation is more likely to occur, as shown in FIG. 2which is a continuous cooling transformation diagram of the eutectoidsteel and the hypereutectoid steel. In other words, the presentinventors have found out that in order to obtain a high strength in theheat-treatment of the hypereutectoid steel rails, an accelerated coolingrate must be increased much more than in the conventional eutectoidcomponent steels. In order to prevent the formation of the proeutecticcementite which causes brittleness as another problem of thehypereutectoid steel, the improvement of the accelerated cooling rate iseffective. As a result, the present inventors have found out that theimprovement in the wear resistance due to a higher carbon content can beexpected by preventing the formation of the pro-eutectic cementite ofthe austenite grain boundary.

Further, the present inventors have experimentally confirmed that thewear resistance of the gage corner portion, which has been a problem inthe conventional rail of the eutectoid carbon-containing steel whichprovides a difference in the hardness inside the section of the headportion, can be further improved by forming the difference in thehardness at the rail head portion having the pearlite structure with theincreased carbon content described above in such a manner that thehardness of the gage corner portion becomes higher than that of the headtop portion, fitting between the wheels and the rails under the initialwear state can be promoted at the same time by reducing the contactsurface pressure and the wear of the head top porion, and buildup of therolling fatigue layer can thus be prevented. The effect brought forth bysetting the hardness of the head top portion to a lower level than thehardness of the gage corner portion is that the cutting work becomeseasier when rail head profile grinding is conducted so as to prevent thelocal wear of the gage corner portion of the outer track rail and toprevent the internal fatigue damage due to the stress concentration onthe inside of the corner portion as has been periodically conducted onheavy load railways. This effect can be similarly obtained when cuttingof the head top portion of the inner track rail is conducted.

The present invention is directed to improve wear resistance and thedamage resistance, as required for the rails of the sharply curved zoneof the heavy load railway, to drastically improve the service life ofthe rails and to provide such rails at a reduced cost.

In the case of resistance flash butt welding which has gained a wideapplication in rail welding, the base metal portion having a highstrength by heat-treatment is softened at the joint portion due to theheat-treatment to thereby invite a local wear, and the drop of the jointportion not only results in the source of occurrence of noise andvibration but also results in the damage of the road bed and breakage ofthe rails.

The present invention solves the problems described above, and has thegist thereof in the following points.

(1) A pearlitic steel rail, having a good wear resistance, comprisingmore than 0.85 to 1.20%, in terms of percentage by weight, of carbon,wherein the structure of the steel rail is a pearlite, a pearlitelamella space of the pearlite is not more than 100 nm, and a ratio ofthe cementite thickness to the ferrite thickness in the pearlite is atleast 0.15.

(2) A pearlitic steel rail, having a good wear resistance, comprisingmore than 0.85 to 1.20%, in terms of percentage by weight, of carbon,and having a good wear resistance, wherein the structure within therange of a depth of 20 mm from the surface of a rail head portion of thesteel rail with the surface of the head portion being the start point isthe pearlite, a pearlite lamella space of the pearlite is not more than100 nm, and a ratio of the cementite thickness to the ferrite thicknessin the pearlite is at least 0.15.

(3) A pearlitic steel rail having a good wear resistance, comprising, interms of percent by weight:

-   -   C: more than 0.85 to 1.20%    -   Si: 0.10 to 1.00%,    -   Mn: 0.40 to 1.50%, and    -   the balance consisting of Fe and unavoidable impurities,        wherein the structure of the steel rail is pearlite, a pearlite        lammella space of the pearlite is not more than 100 nm, and a        ratio of the cementite thickness to the ferrite thickness in the        pearlite is at least 0.15.

(4) A pearlitic steel rail having a good wear resistance, comprising, interms of percent by weight:

-   -   C: more than 0.85 to 1.20%,    -   Si: 0.10 to 1.00%,    -   Mn: 0.04 to 1.50%, and    -   the balance consisting of Fe and unavoidable impurities,        wherein the structure within the range of the depth of 20 mm        from the surface of a head portion of the steel rail with the        surface of the rail head portion being the start point is the        perlite, a pearlite lamella space of the pearlite is not more        than 100 nm, and a ratio of the cementite thickness to the        ferrite thickness in the pearlite is at least 0.15.

(5) A pearlitic steel rail having a good wear resistance, comprising, interms of percent by weight:

-   -   C: more than 0.85 to 1.20%,    -   Si: 0.10 to 1.00%,    -   Mn: 0.40 to 1.50%,    -   at least one of the members selected from the group consisting        of:        -   Cr: 0.05 to 0.50%,        -   Mo: 0.01 to 0.20%,        -   V: 0.02 to 0.30%,        -   Nb: 0.002 to 0.05%,        -   Co: 0.10 to 2.00%,        -   B: 0.0005 to 0.005%, and        -   the balance consisting of Fe and unavoidable impurities,            wherein the structure of the steel rail is pearlite, a            pearlite lamella space of the pearlite is not more than 100            nm, and a ratio of the cementite thickness to the ferrite            thickness in the pearlite structure is at least 1.15.

(6) A pearlitic steel rail having a good wear resistance, comprising, interms of percent by weight:

-   -   C: more than 0.85 to 1.20%.    -   Si: 0.10 to 1.00%,    -   Mn: 0.40 to 1.50%,    -   at least one of the members selected from the group consisting        of:        -   Cr: 0.05 to 0.50%,        -   Mo: 0.01 to 0.20%,        -   V: 0.02 to 0.30%,        -   Nb: 0.002 to 0.05%,        -   Co: 0.10 to 2.00%,        -   B: 0.0005 to 0.005%, and        -   the balance consisting of Fe and unavoidable impurities,            wherein the structure of the steel rail within the range of            the depth of 20 mm from the surface of a head portion of the            steel rail with the surface of the rail head portion being            the start point is the pearlite, a pearlite lamella space of            the pearlite is not more than 100 nm, and a ratio of the            cementite thickness to the ferrite thickness in the pearlite            is at least 0.15.

(7) A pearlitic steel rail having a good weldability and a high wearresistance according to the item (1) or (2), wherein the differencebetween the hardness of a weld joint portion and a base metal is notmore than Hv 30.

(8) A pearlitic steel rail having a good weldability and a good wearresistance according to any of the items (3) to (6), wherein thechemical components further satisfy the relation Si+Cr+Mn: 1.5 to 3.0%in terms of percent by weight.

(9) A method of producing a pearlitic steel rail having a good wearresistance, comprising the chemical components according to any of theitems (1) to (6), which comprises the steps of hot rolling a melted andcast steel, acceleratedly cooling a steel rail holding a rolling heatimmediately after hot rolling or a steel rail heated for the purpose ofheat-treatment at a cooling rate of 1° to 10° C./sec from an austenitetemperature, stopping accelerated cooling when the steel railtemperature reaches 700° to 500° C., and thereafter leaving the steelrail to cool, wherein the hardness within the range of the depth of 20mm from the surface of a head portion of the steel rail is at least Hv320.

(10) A method of producing a pearlitic steel rail having a good wearresistance, comprising the chemical components according to any of theitems (1) to (6), which comprises the steps of hot rolling a melted andcast steel, acceleratedly cooling a steel rail holding a rolling heatimmediately after hot rolling or a steel rail heated for the purpose ofheat-treatment at a cooling rate of more than 10° to 30° C./sec from anaustenite temperature, stopping accelerated cooling when pearlitetransformation of the steel rail proceeds at least 70%, and thereafterleaving the steel rail to cool, wherein the hardness within the range ofthe depth of 20 mm from the surface of a head portion of the steel railis at least Hv 320.

(11) A method of producing a pearlitic steel rail having a good wearresistance and a good damage resistance, comprising the chemicalcomponents according to any of the items (1) to (6), which comprises thesteps of hot rolling a melted and cast steel, acceleratedly cooling asteel rail holding a rolling heat immediately after hot rolling or agage corner portion of a steel rail heated for the purpose ofheat-treatment at a cooling rate of 1° to 10° C./sec, from an austenitetemperature, stopping accelerated cooling when the temperature of thegage corner portion of the steel rail reaches 700° to 500° C., andthereafter leaving the steel rail to cool, wherein the hardness of thegage corner portion of the steel rail is at least Hv 360 and thehardness of a head top portion is Hv 250 to 320.

(12) A method of producing a pearlitic steel rail having a good wearresistance and a good damage resistance, comprising the chemicalcomponents according to any of the items (1) to (6), which comprises thesteps of hot rolling a melted and cast steel, acceleratedly cooling asteel rail holding a rolling heat immediately after hot rolling or agage corner portion of a steel rail heated for the purpose ofheat-treatment at a cooling rate of more than 10° to 30° C./sec from anaustenite temperature, stopping accelerated cooling when a pearlitetransformation of the gage corner portion of the steel rail proceeds atleast 70%, and thereafter leaving the steel rail to cool, wherein thehardness of the gage corner portion of the steel rail is at least Hv 360and the hardness of the head top portion is Hv 250 to 320.

(13) A method of producing a pearlitic steel rail having goodweldability and a good wear resistance, according to the item (7) or(8), which comprises the steps of hot rolling a melted and cast steel,acceleratedly cooling a steel rail holding rolling heat immediatelyafter hot rolling or a steel rail heated for the purpose ofheat-treatment at a cooling rate of 1° to 10° C./sec from an austenitetemperature, stopping accelerated cooling when the steel railtemperature reaches 700° to 500° C., and thereafter leaving the steelrail to cool, wherein the hardness within the range of the depth of 20mm from the surface of a head portion of the steel rail is at least Hv320.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing wear test characteristics, determined by aNishihara wear tester, of a conventional eutectoid component pearliterail and of a hypereutectoid component pearlite rail steel according tothe present invention.

FIG. 2 is a diagram showing continuous cooling transformation of aneutectoid rail steel and of a hypereutectoid rail steel after heating at1,000° C.

FIG. 3 is a diagram showing the relation between a lamella space and acementite thickness/ferrite thickness between a comparative rail steeland a rail steel according to the present invention.

FIG. 4 is a diagram showing the relation between the lamella space and awear amount as the wear test result of a comparative rail steel and of arail steel according to the present invention.

FIG. 5 is a photograph showing an example of the space between thecementite/ferrite layers in the rail steel according to the presentinvention.

FIG. 6 is a schematic view showing the names of surface positions in thesection of a rail head portion.

FIG. 7 is a schematic view showing a Nishihara wear tester.

FIG. 8 is a diagram showing the relation between the hardness and thewear amount as the wear test results of the rail steel according to thepresent invention and of the comparative rail steel.

FIG. 9 is a diagram showing an example of the hardness distribution ofthe section of the rail head portion according to an embodiment of thepresent invention.

FIG. 10 is a schematic view showing the outline of a rolling fatiguetester.

FIG. 11 is a diagram showing the relation between the hardness of thegage corner portion and the wear amount in the rolling fatigue test.

FIG. 12 is a diagram showing the relation between the position in theproximity of a weld portion and hardness distribution of the rail steelaccording to the present invention and of a comparative rail steel.

BEST MODE FOR CARRYING OUT THE INVENTION

The pearlite structure of the eutectoid carbon component that has beenused as the rail steel in the past has a lameller structure comprising aferrite layer having a low hardness and a tabular hard cementite layer.A method of improving the wear resistance of the pearlite structuregenerally reduces the lamella space: λ [λ=(ferrite thickness:t₁)+(cementite thickness: t₂)] and increases the hardness. As shown inFIG. 1 on page 1217 of Metallurgical Transactions, Vol. 7A (1976), forexample, the hardness can be greatly improved by rendering the lamellaspace in the pearlite structure fine.

In the high hardness rails exhibiting the fine pearlite structure ofeutectoid carbon steel, the hardness of the existing pearlite is theupper limit. When attempts are made to further make fine the pearlitelamella space by increasing the cooling rate in heat-treatment or byadding alloys, a hard martensite structure is formed inside the pearlitestructure, so that both the toughness and the wear resistance of therail drop.

Another solution method would be one that uses a material having ametallic structure which has a better wear resistance than that of thepearlite structure. In the case of rolling wear between the rails andthe wheels, however, materials which are more economical and have abetter wear resistance than the fine pearlite structure have not yetbeen found.

The wear mechanism of the pearlite structure is as follows. In the railsurface layer with which the wheel comes into contact, the work layerreceiving repetitive contact with the wheel first undergoes plasticdeformation in the opposite direction to the travelling direction of thetrain, and the soft ferrite layer sandwiched between the cementiteplates is squeezed out and at the same time, the cementite plates arecut off upon receiving the work. Further, the cut cementite changes tospheres by receiving repeatedly the load of the wheel, and only the hardcementites are thereafter piled up immediately below the rolling surfaceof the wheel. In addition to work hardening by the wheel, the density ofthis cementite plays an important role in securing the wear resistance,and this fact is confirmed by experiment. Therefore, the inventors ofthe present invention make the pearlite lamella space fine in order toobtain the strength (hardness) and at the same time, increase the ratioof the tabular hard cementite structure which secures the wearresistance of the pearlite structure, by increasing the carbon content.In this way, the cementite becomes more difficult to be cut off evenwhen receiving work and to become spheres. The present inventors haveconfirmed through experiments that the wear resistance can bedrastically improved, without spoiling the toughness and ductility, byincreasing the cementite density immediately below the rolling surface.

Hereinafter, the present invention will be explained in further detail.

To begin with, the reasons why the chemical components of the rail arelimited as described above in the present invention will be explained.

Carbon is an effective element for generating the pearlite structure andsecuring the wear resistance. Generally, 0.60 to 0.85% of C is used forthe rail steel. If the C content is not more than 0.85%, the ratio Rc(Rc=t₂/t₁) of the cementite thickness t₂ to the ferrite thickness t₁ inthe pearlite structure, which secures the wear resistance, of at least0.15 cannot be secured, and furthermore, the lamella space cannot bekept below 100 nm in the pearlite structure due to the drop ofhardenability. If the C content exceeds 1.20%, the quantity ofpro-eutectic cementite of the austenite grain boundary increases andboth ductility and toughness greatly drop. Therefore, the C content islimited to the range of more than 0.85 to 1.20%.

Next, elements other than C will be explained.

Silicon is the element which improves the strength by solid solutionhardening to the ferrite phase in the pearlite structure and, thoughlimitedly, it improves toughness of the rail steel. If the Si content isless than 0.10%, its effect is not sufficient, and when the Si contentexceeds 1.20%, it invites brittleness and a drop of weldability.Therefore, the Si content is limited to 0.10 to 1.20%.

Manganese is the element which similarly lowers the pearlitetransformation temperature, contributes to a higher strength byincreasing hardenability, and restricts the formation of thepro-eutectic cementite. If the Mn content is less than 0.40%, the effectis small and if it exceeds 1.50%, a martensite structure is likely to beformed at the segregation portion. Therefore, the Mn content is limitedto 0.40 to 1.50%.

Further, at least one of the following elements is added, whenevernecessary, to the rail produced by the component composition describedabove in order to improve the strength, the ductility and the toughness:

-   -   Cr: 0.05 to 0.50%,    -   Mo: 0.01 to 0.20%,    -   V: 0.02 to 0.30%,    -   Nb: 0.002 to 0.050%,    -   Co: 0.10 to 2.00%,    -   B: 0.0005 to 0.005%.

Next, the reasons, why the chemical components are stipulated asdescribed above will be explained.

Chromium raises the equilibrium transformation point of pearlite andeventually contributes to the higher strength by making the pearlitestructure fine. At the same time, it reinforces the cementite phase inthe pearlite structure and improves the wear resistance. If the Crcontent is less than 0.05%, the effect of Cr is small and if it exceeds0.50%, the excessive addition of Cr invites the formation of themartensite structure and brittleness of the steel. Therefore, the Crcontent is limited to 0.05 to 0.50%.

Molybdenum raises the equilibrium transformation point of pearlite inthe same way as Cr and eventually contributes to the higher strength bymaking the pearlite structure fine. Mo also improves the wearresistance. If the Mo content is less than 0.01%, however, its effect issmall and if it exceeds 0.20%, the excessive addition invites the dropof the pearlite transformation rate and the formation of the martensitestructure which is detrimental to the toughness. Therefore, the Mocontent is limited to 0.01 to 0.20%.

Vanadium improves the plastic deformation capacity by precipitationhardening due to vanadium carbides and nitrides formed during thecooling process at the time of hot rolling, restricts the growth of theaustenite grains when heat-treatment is carried out at a hightemperature to thereby make fine the austenite grains, reinforces thepearlite structure after cooling and improves the strength and thetoughness required for the rail. If the V content is less than 0.03%,its effect cannot be expected and if it exceeds 0.30%, its effect againcannot be expected. Therefore, the V content is limited to 0.03 to0.30%.

Niobium forms niobium carbides and nitrides in the same way as V and iseffective for making the austenite grains fine. The austenite graingrowth restriction effect of Ni lasts to a higher temperature (near1,200° C.) than V, and Nb improves the ductility and the toughness ofthe rail. If the Nb content is less than 0.002%, however, the effect ofNb cannot be expected and if it exceeds 0.050%, the excessive additiondoes not increase the effect. Therefore, the Nb content is limited to0.002 to 0.050%.

Cobalt increases transformation energy of pearlite and improves thestrength by making the pearlite structure fine. If the Co content isless than 0.10%, however, its effect cannot be expected and if itexceeds 2.00%, the excessive addition saturates. Therefore, the Cocontent is limited to 0.10 to 2.00%.

Boron provides the effect of restricting the proeutectic cementiteresulting from the original austenite grain boundary, and is theeffective element for stably forming the pearlite structure. If the Bcontent is less than 0.0005%, however, its effect is weak and if the Bcontent exceeds 0.0050%, coarse B compounds are formed and the railproperties are deteriorated. Therefore, the B content is limited to0.0005 to 0.0050%.

In connection with the improvement in the weld portion, the presentinvention pays specific attention to Si, Cr and Mn as the railcomponents in order to prevent the drop of the hardness of the jointportion which occurs at the time of welding of the conventional railsteels at the time of flash butt welding, etc., in the hardnessdistribution of the weld joint portion. In other words, the drop of thehardness of the joint portion by flash butt welding, etc., brings thehardness of not greater than Hv 30 for the base metal, and if theSi+Cr+Mn value in this instance is less than 1.5%, the drop of thehardness of the weld joint portion cannot be prevented. If the Si+Cr+Mnvalue is greater than 3.0%, on the other hand, the martensite structuremixes into the weld joint portion, and the properties of the jointportion are deteriorated. Therefore, the Si+Cr+Mn value is limited to1.5 to 3.0% in the present invention.

The rail steel having the component composition described above ismelted by a melting furnace used ordinarily such as a converter, anelectric furnace, etc., and the rail is produced by subjecting thismolten steel to ingot making, breakdown method or a continuous castingmethod, and further to hot rolling. Next, the head portion of the railholding the high temperature heat of hot rolling or the head portion ofthe rail heated to a high temperature for the purpose of heat-treatmentis acceleratedly cooled, and the lamella space of the pearlite structureof the rail head portion is made fine.

Next, the range in which the pearlite structure is secured is preferablyset to the range of the depth of at least 20 mm from the surface of therail head portion with this rail head portion being the start point, forthe following reason. For, if the depth is less than 20 mm, thewear-resistance range of the rail head portion is small and longerservice life of the rail cannot be obtained sufficiently. If the rangein which the pearlite structure is secured is greater than the range ofthe depth of 30 mm from the rail head surface with this rail headsurface being the start point, desired longer service life of the railcan be obtained sufficiently.

By the way, the term “rail head surface” means the rail head top portionand the rail head side portion or in other words, the portion where thewheel tread surface and the flange of the train come into contact withthe rail.

Next, the reason why the pearlite lamella space λ (λ=ferrite thicknesst₁+cementite thickness t₂) and the ratio R_(c) (R_(c)=t₂/t₁) of thecementite thickness t₂ to the ferrite thickness t₁ in the pearlitestructure are limited as described above will be explained.

First, the reason why the pearlite lamella space is limited to notgreater than 100 nm will be explained.

When the lamella space is greater than 100 nm, it becomes difficult tosecure the hardness of the pearlite structure, and even when the ratioR_(c) (R_(c)=t₂/t₁) of the cementite thickness of at least 0.15 issecured, the wear resistance required for the rail on the sharp curve ofthe heavy load railway having a wheel weight as great as 15 tons cannotbe secured. Since surface damage such as creak crack resulting fromplastic deformation is induced on the rail head surface, the pearlitelamella space λ is limited to not greater than 100 nm.

Next, the reason why the ratio R_(c) (R_(c)=t₂/t₁) of the cementitethickness t₂ to the ferrite thickness t₁ in the pearlite structure islimited to at least 0.15 is as follows. If R_(c) is not greater than0.15, it becomes difficult to secure the strength of cementite(resistance to separating and sphering) immediately below the rollingsurface which secures the wear resistance of the pearlite steel, and toimprove the cementite density, and the improvement in the wearresistance cannot be recognized in comparison with the conventionaleutectoid rails. Therefore, R_(c) is limited to at least 0.15.

By the way, the pearlite Lamella space λ, the ferrite thickness t₁ andthe cementite thickness t₂ are measured in the following way. A sampleis first etched by a predetermined etching solution such as nital orpicral, and in some cases, two-stage replicas are collected from thesurface of the etched sample. The sample is inspected in 10 fields by ascanning electron microscope, and λ, t₁ and t₂ are measured in eachvisual field. The measurement values so obtained are then averaged.

Though the metallic structure of the rail is preferably the pearlitestructure, a trace amount of proeutectic cementite is sometimes formedin the pearlite structure depending on the cooling method of the rail oron the segregation state of the raw materials. Even when a trace amountof pro-eutectic cementite is formed in the pearlite structure, it doesnot exert a great influence on the wear resistance, the strength and thetoughness of the rail. For this reason, the structure of the pearliticsteel rail according to the present invention may contain a considerableamount of pro-eutectic cementite in mixture.

Next, the hardness at each rail portion in the present invention will beexplained.

FIG. 6 shows the names of the surface positions in the section of thehead portion of the rail in the present invention. The rail head portionincludes a head top portion 1 and head corner portions 2. A part of oneof the head corner portions 2 is a gage corner portion (G.C. portion)which mainly comes into contact with the wheel flange.

The preferred range of the hardness of the pearlite structure accordingto the present invention is at least Hv 320. If the hardness is lessthan Hv 320, it becomes difficult to secure the wear resistance requiredfor the rail of the heavy load railway by the present component system,and a metallic plastic flow occurs due to strong contact between therail and the wheel at the rail G.C. (gage corner) portion in the sharplycurved zone, so that surface damage such as head check or flakingoccurs.

In order to further improve the damage resistance of the gage cornerportion described above, the hardness of the rail gage corner portion ispreferably at least Hv 360 when the damage of the corner portion isconsidered in the present invention. If the hardness is less than Hv360, it is difficult to secure the wear resistance required for the gagecorner portion of the rail in the sharply curved zone of the heavy loadrailway by the component system of the present invention. Further,metallic plastic flow occurs due to the strong contact between the railand the wheel at the G.C. portion, and surface damage such as head checkor flaking thereby occurs.

Improving the strength of the gage corner portion is also effective forpreventing the damage due to the internal fatigue that occurs frominside the corner portion, and the higher hardness obtained by a highercarbon content can prevent the formation of the pro-eutetic ferrite asone of the start points of internal fatigue damage. From these twoaspects, too, not only the wear but also the internal fatigue damage canbe improved and the longer service life can be accomplished.

In this case, the hardness of the rail head top portion is preferably Hv250 to 320. If the hardness is less than Hv 250, accumulation of therolling fatigue layer by the reduction of the contact surface pressureand the promotion of the wear can be prevented, but the strength of thetop head portion is remarkably insufficient. Therefore, damage resultingfrom plastic deformation such as head checkproceeds remarkably beforethe rolling fatigue layer is removed by the wear and furthermore,corrugated wear is induced. Therefore, the hardness of the head topportion is limited to at least Hv 250. If the hardness exceeds Hv 320,the reduction of the contact surface pressure of the rail head topportion and the promotion of the wear become insufficient, and therolling fatigue layer is built up at the head top portion.

Here, when the service life of the rail due to the wear is taken intoconsideration, the range of the depth of at least 20 mm from the surfaceof each portion as the start point preferably has a predeterminedhardness as to the hardness of the gage corner portion and the head topportion.

Next, the reason why the cooling stop temperature range and theaccelerated cooling rate are limited as described above will beexplained in detail.

First, accelerated cooling from the austenite zone temperature islimited to the cooling rate of 1° to 10° C./sec and the cooling stoptemperature is limited to the range of 700° to 500° C., for thefollowing reasons.

When accelerated cooling is stopped at a temperature higher than 700°C., the pearlite transformation starts occurring immediately afteraccelerated cooling, and a coarse pearlite structure having a lowhardness is formed, so that the hardness of the rail head portionbecomes less than Hv 320. Therefore, it is limited to a temperature nothigher than 700° C. When accelerated cooling is carried out down totemperature less than 500° C., on the other hand, sufficientrecuperation from inside the rail cannot be expected after acceleratedcooling, and the martensite structure detrimental to the toughness andthe wear resistance of the rail is formed at the segregation portion.Therefore, it is limited to a temperature not lower than 500° C. Thetechnical significance that the cooling stop temperature is at least500° C. is that the microsegregation portion inside the rail isconverted to a sound pearlite structure. and at least 90% of the railhead portion as a whole has completed the pearlite transformation.

When the accelerated cooling rate is less than 1° C./sec, the pearlitetransformation starts occurring during accelerated cooling. Inconsequence, a coarse pearlite structure having a low hardness is formedand the hardness of the rail head portion is less than Hv 320. Further,large quantities of pro-eutectic cementite detrimental to the toughnessand the ductility of the rail are formed. Therefore, the acceleratedcooling rate is limited to at least 1° C./sec. A cooling rate exceeding10° C./sec cannot be accomplished by using air which is the mosteconomical and the most stable cooling medium from the aspect ofheat-treatment. Therefore, the cooling rate is limited to 10° C./sec.

In order to produce a rail having a pearlite structure having a hardnessof at least 320 and a high wear resistance, therefore, acceleratedcooling must be carried out at a rate of 1° to 10° C./sec from theaustenite zone temperature to the cooling stop temperature of 700° to500° C., and a pearlite structure having a high hardness is preferablyformed in a low temperature zone.

Next, accelerated cooling, when a cooling medium other than water suchas mist, atomized water, etc., is used, is set to a cooling rate of morethan 10° to 30° C./sec from the austenite temperature zone, and isstopped at the point when the pearlite transformation has proceeded atleast 70%, for the following reasons.

First, it can be appreciated from FIG. 2 that the composition alwayspasses through the pearlite nose at the cooling rate of not higher than10° C./sec, but only those having a limited C % pass through the noseposition below 10° C./sec. In the latter case, supercooling becomesgreater with a higher cooling rate, and if cooling is as such continued,large quantities of martensite structure mix into the pearlitestructure. If supercooling is great, on the other hand, the pearlitetransformation of the rail head portion can be completed as a whole byexothermy of the pearlite transformation even when cooling is stopped ata certain temperature, provided that the pearlite transformation hasproceeded to a predetermined extent. The limit pearlite transformationquantity for completing the pearlite transformation is at least 70% onthe basis of the detailed experiments, and the example of 0.95% shown inFIG. 2 is conceptually shown in super-position with the CCT diagram. Itcan be understood from the diagram that when a 75% transformation pointis reached, the passage through the pearlite transformation zone can beaccomplished by recuperation by stopping accelerated cooling, causingrecuperation in the rail itself and bringing the cooling characteristicas dose as possible to the cooling curve of not greater than 10° C./sec.

This point will be explained below in further detail.

First, the reason why the cooling rate is limited to more than 10° to30° C./sec from the austenite zone temperature when water, etc., is usedas the cooling medium is as follows. In this case, the productivity ofheat-treatment is by far higher than when cooling is carried out at arate of 1° to 10° C./sec, and as shown in the continuous coolingtransformation diagram of FIG. 2, the pearlite nose shifts to theshorter time side in the hyper-eutectoid rail steel than in theeutectoid rail. The nose position corresponds to the rate of more than10° to 30° C./sec in the component range of the present invention. Incontinuous cooling, pearlite transformation heat is forcedly restricted,and when cooling is, as such, carried out at a predetermined rate, themartensite structure mixes into the pearlite structure. In the practicalheat-treatment of the rails, however, the pearlite transformation issufficiently promoted by the mass of the rail once the pearlitetransformation nose is reached by the volume of the rail head portion.Because the water quantity adjustment at a rate of lower than 10° C./seccannot stably control cooling when the cooling medium such as water isused, the lower limit is limited to 10° C./sec. When cooling is carriedout at a cooling rate exceeding 30° C./sec, the composition does not hitthe pearlite nose and the major proportion is converted to themartensite structure. Even when it reaches the pearlite nose, pearlitetransformation of more than 70% cannot be expected, and the pearlitetransformation remains insufficient and the martensite structure mixesafter cooling.

The reason why cooling is stopped at the pearlite transformation of atleast 70% is because, if accelerated cooling at a rate of more than 10°to 30° C./sec is continued down to a low temperature, completion of thepearlite transformation of the rail head portion as a whole cannot beaccomplished even when exothermy by the pearlite transformation bystopping cooling is taken into consideration. As a result, largequantities of martensite are formed in the rail head portion but theinside the rail head portion in which microscopic segregation exists iscooled while it does not yet undergo transformation, so that island-likemartensite structures exist in the spot form and they are detrimental tothe rail. Therefore, it is necessary to stop accelerated cooling at thepoint when at least 70% of pearlite transformation is formed inside thepearlite nose and to sufficiently promote the pearlite transformation bythe heat of the rail head portion. Here, the scale for judging at least70% of the pearlite transformation is as follows. Namely, when thecooling rate is measured by a thermo-couple fitted to the surface of therail head portion, exothermy of the pearlite transformation occurs, anda point immediately before the point at which the temperature rise dueto exothermy by the transformation stops corresponds to about 70% ofpearlite transformation quantity.

The range of the accelerated cooling rate is limited to more than 10° to30° C./sec from the concept of the accelerated cooling rate and the stoptiming of accelerated cooling described above, and the stop timing ofthe accelerated cooling is limited to at least 70% of the pearlitetransformation. Incidentally, means for obtaining the cooling rate ofmore than 10° to 30° C./sec is mist cooling, water-air mixture spraycooling or their combination, or immersion of the rail head portion orthe whole into oil, hot water, polymer plus water, salt bath, etc.

After accelerated cooling is stopped, gradual cooling is carried out byleaving the rail standing. The cooling rate at this time is generallynot higher than 1° C./sec, and the martensite transformation does notpractically occur even at a low temperature.

By the way, the object of improving the weld portion according to thepresent invention can be sufficiently accomplished by setting thecooling rate of accelerated cooling to 1° to 10° C./sec and stoppingaccelerated cooling at a temperature of 700° to 500° C. Further, theimprovement of the damage resistance of the gage corner portion can beaccomplished by satisfying the accelerated cooling condition describedabove.

Hereinafter, the present invention will be explained in further detailwith reference to Examples thereof shown in the accompanying drawings.

EXAMPLES Example 1

Table 1 tabulates the chemical components of the rail steel having thepearlite structure of this Example 1 of the present invention and thechemical components of a Comparative rail steel. Table 2 tabulates thelamella space λ (λ=ferrite thickness t₁+cementite thickness t₂), theratio R_(c) (R_(c)=t₂/t₁) and the result of measurement of the wearquantity after repetition of 500,000 times under a dry condition by aNishihara type wear test of each of these steels.

Further, FIGS. 3 and 4 show the relation between the lamella space (λ)and the ratio of the cementite thickness to the ferrite thickness andthe relation between the lamella space (λ) and the wear quantity of theComparative rail steel and the present rail steel. FIG. 5 shows a10,000× micrograph of the present rail steel (No. 8). FIG. 5 is obtainedby etching the present rail steel by a 5% nital solution and observingit through a scanning electron micrograph. A white portion in thedrawing represents the cementite layer and a black portion representsthe ferrite layer.

Incidentally, the construction of the rails is as follows. Rails of thisinvention (10 steels, Nos. 1 to 10)

Heat-treated rails applied with accelerated cooling at the head portionthereof and having the components within the range described above, apearlite lamella space λ (λ=ferrite thickness t₁+cementite thickness t₂)of not more than 100 nm and a ratio R_(c) (R_(c)=t₂/t₁) of the cementitethickness t₂ to the ferrite thickness t₁ of at least 0.15 in thepearlite structure.

Comparative rails (6 rails, Nos. 11 to 16)

Comparative rails by eutectoid carbon-containing rails

The wear testing condition is as follows. FIG. 7 shows the Nishiharatype wear testing machine. In this drawing, reference numeral 3 denotesa rail testpiece, 4 denotes a mating material and 5 denotes a coolingnozzle.

-   -   Testing machine: Nishihara type wear tester    -   Shape of testpiece: disc-like testpiece (outer diameter=30 mm,        thickness=8 mm)    -   Test load: 686N    -   Slippage ratio: 9%    -   Wheel material: tempered martensite steel (Hv 350)    -   Atmosphere: in air    -   Compulsive cooling by compressed air (flow rate: 100 Nl/min)    -   Number of times of repetition: 700,000 times

TABLE 1 chemical composition (wt %) rail No. C Si Mn Cr Mo V Nb CoPresent 1 0.86 0.52 1.20 — 0.19 — — — steel 2 0.86 0.61 1.21 — — — —1.20 3 0.90 0.25 1.12 — — — — — 4 0.91 0.25 0.81 0.45 — — — — 5 0.940.25 0.85 — — — — — 6 0.95 0.21 0.61 0.30 — — — — 7 0.97 0.25 0.75 — — —— — 8 0.99 0.17 0.49 0.23 — — — — 9 1.05 0.20 0.59 — — — 0.05 — 10 1.190.10 0.40 — — 0.17 — — Com- 11 0.78 0.24 1.33 — — — — — para- 12 0.790.50 1.24 — — — — — tive 13 0.78 0.81 1.11 — — — — — rail 14 0.79 0.241.10 0.21 — — — — steel 15 0.79 0.50 1.03 0.24 — — — — 16 0.78 0.81 0.910.58 — — — —

TABLE 2 lamella space wear amount rail No. λ (nm) R_(c=t2)/t₁*(g/500,000 times) Present 1 85 0.15 0.76 rail steel 2 99 0.16 0.73 3 900.17 0.66 4 82 0.17 0.62 5 92 0.18 0.61 6 80 0.18 0.58 7 87 0.19 0.56 877 0.19 0.51 9 72 0.20 0.49 10 68 0.24 0.48 Comparative 11 121 0.13 1.31rail steel 12 110 0.14 1.21 13 105 0.13 1.18 14 86 0.14 1.02 15 84 0.140.98 16 79 0.13 0.94 *ratio R_(c) = cementite thickness t₂: ferritethickness t₁

As can be seen from Tables 1 and 2, the present rail steels make finethe lamella space (λ) and at the same time, increase the ratio R_(c)(R_(c)=t₂/t₁) of the cementite thickness t₂ to the ferrite thickness t₁much more than the Comparative rail steels. Therefore, the presentsteels have a smaller wear amount at the same lamella space than theComparative rail steels and have drastically improved wear resistance.

Example 2

Table 3 shows the chemical components of the Present rail steels and theaccelerated cooling condition, and Table 4 shows the chemical componentsof the Comparative rail steels and the accelerated cooling condition.Further, Tables 3 and 4 represent also the hardness after acceleratedcooling and the measurement result of the wear amount after repetitionof 700,000 times under the compulsive cooling condition by compressedair in the Nishihara type wear test shown in FIG. 7.

FIG. 8 graphically compares the wear test results between the Presentrail steels and the Comparative rail steels shown in Tables 1 and 4 interms of the relation between the hardness and the wear amount.

By the way, the rail construction is as follows.

Present rails (16 rails) Nos. 17 to 32

Heat-treated rails having the components within the range describedabove, and exhibiting the pearlite structure within the range of depthof at least 20 mm from the surfaces of the gage corner portion and thehead top portion of the steel rails as the start point, and applied withaccelerated cooling at the head portion having the hardness of at leastHv 320 in the pearlite structure within the range described above.

Comparative rails (6 rails) Nos. 33 to 38

TABLE 3 accelerated wear amount cooling hardness of rail head rate ofhead of head portion chemical composition (wt %) portion portiontestpiece rail No. C Si Mn Cr Mo V Nb Co B (° C./sec) (Hv) (g/700,000times) rail of 17 0.86 0.49 1.48 — 0.02 — — — — 4 385 0.90 this 18 0.880.65 1.05 — — — — 0.05 — 10 391 0.86 invention 19 0.90 0.49 1.02 0.21 —— — — — 3 402 0.81 20 0.91 0.98 0.81 0.59 — — — — — 1 412 0.74 21 0.940.25 0.85 — — 0.09 — — — 5 401 0.68 22 0.95 0.24 0.83 — — 0.10 — — — 5400 0.68  319* 23 0.94 036 0.86 — — 0.08 — — — 5 398 0.70  275* 24 0.950.21 0.61 0.30 — — — — — 4 415 0.54 25 0.94 0.22 0.63 0.29 — — — — — 4413 0.55  317* 26 0.94 0.23 0.61 0.29 — — — — — 4 410 0.57  278* 27 0.970.46 0.75 — — — — — — 2 371 0.52 28 0.98 0.43 0.73 — — — — — — 2 3690.52  316* 29 0.97 0.45 0.75 — — — — — — 2 368 0.54  276* 30 0.98 0.170.49 0.23 — — — — — 3 384 0.44 31 1.04 0.22 0.60 — — — 0.05 — — 3 4160.31 32 1.19 0.10 0.41 — — — — — 0.0010 2 421 0.21 *hardness at a pointof 1 mm below a sole surface when a sole was cooled under control.

TABLE 4 accelerated wear amount cooling hardness of rail head rate ofhead of head portion chemical composition (wt %) portion portiontestpiece rail No. C Si Mn Cr Mo V Nb Co B (° C./sec) (Hv) (g/700,000times) Comparative 33 0.77 0.22 1.36 — — — — — — 4 364 1.44 rail steel34 0.78 0.54 1.30 — — — — — — 3 368 1.40 35 0.82 0.78 1.05 — — — — — — 3374 1.32 36 0.81 0.21 1.21 0.19 — — — — — 3 386 1.22 37 0.82 0.49 1.100.22 — — — — — 3 396 1.17 38 0.81 0.85 0.81 0.51 — — — — — 4 412 1.11

As shown in FIG. 8, the Present rail steels increase the carbon contentin comparison with the Comparative rail steels and at the same time,improve the hardness. In this way, the present rail steels have asmaller wear amount at the same hardness but have drastically improvedwear resistance.

Example 3

Table 5 tabulates the chemical components, the accelerated cooling rateat the time of heat-treatment of the rails and the pearlite structurefractions at the stop of accelerated cooling of each of the present railsteels and Comparative rail steels. Further, Table 6 tabulates thehardness (Hv) of the head surface after heat-treatment of the rails andthe wear amount after the Nishihara type wear test of each of thepresent rail steels and the Comparative rail steels. The wear testresults of the rail head materials by the Nishihara type wear testershown in FIG. 7 are shown.

By the way, the wear testing condition are as follows.

-   -   Testing machine: Nishihara type wear tester    -   Shape of testpiece: disc-like testpiece (outer diameter: 30 mm,        thickness: 8 mm)    -   Test load: 686N    -   Slippage ratio: 20%    -   Wheel material: pearlite steel (Hv 390)    -   Atmosphere: in air (compulsive cooling by compressed air)    -   Number of times of repetition: 700,000 times

TABLE 5 head pearlite portion proportion accelerated at stop of chemicalcomposition (wt %) cooling rate cooling rail No. C Si Mn Cr Mo V Nb (°C./sec) (%) Present 39 0.86 0.86 1.20 28 75 rail steel 40 0.90 0.63 1.0025 80 41 1.02 0.45 0.81 20 85 42 1.20 0.31 0.62 15 90 43 1.39 0.21 0.2412 95 44 0.87 0.23 0.45 0.55 25 75 45 0.91 0.23 0.40 0.25 0.21 20 75 460.89 0.41 0.51 0.12 30 80 47 0.92 0.56 0.65 0.08 0.015 30 80 Comparative48 0.76 0.23 0.89 25 95 rail steel 49 0.79 0.41 0.87 0.25 28 90 50 0.760.82 0.88 055 15 85 51 1.50 0.23 0.85 12 *—  52 0.90 1.23 0.85 12 *65 53 0.87 0.23 1.82 12 *70  *Martansite structure and bainite structuremixed into the rail head portion after cooling.

TABLE 6 hardness of head portion wear amount rail No. (Hv) (g/700,000times) Present rail 39 403 0.95 steel 40 395 0.92 41 418 0.63 42 4310.25 43 438 0.21 44 396 0.98 45 403 0.74 46 392 0.75 Comparative 47 3970.77 rail steel 48 385 1.36 49 391 1.25 50 393 1.23 51 580 1.56 52 3711.35 53 395 1.31

In comparison with the eutectoid pearlite steels according to the priorart, the hypereutectoid pearlite rails according to the presentinvention have a higher wear resistance at the same hardness,drastically improve the wear resistance of the outer track rail of thecurved zone, have a high internal fatigue damage resistance because theformation of the pro-eutectic ferrite as the start point of the internalfatigue cracks formed inside the gage corner portion of the outer trackrail laid down in the sharp curve zone does not exist, and drasticallyimprove the rail heat-treatment properties by the combination of quickaccelerated cooling and the stop of cooling.

Example 4

Table 7 tabulates the chemical components of each of the present railsteels and the Comparative rail steels. Table 8 tabulates theaccelerated cooling rate of the rail gage corner portions, and thehardness of the gage corner portion and the head top portion. FIG. 9shows an example of the hardness distribution of the section of the headportion of the present rail (No. 46).

TABLE 7 chemical composition (wt %) rail No. C Si Mn Cr Mo V Nb Co BPresent rail steel 54 0.87 0.51 1.49 — 0.01 — — — — 55 0.88 0.67 1.01 —— — — 0.40 — 56 0.90 0.55 0.98 0.21 — 0.07 — — — 57 0.91 0.99 0.78 0.58— — — — — 58 0.94 0.26 0.88 — — — — — 0.0010 59 0.95 0.22 0.71 0.25 — —— — — 60 0.97 0.49 0.78 — — — — — — 61 0.98 0.19 0.51 0.23 — — — — — 621.05 0.30 0.71 — — — 0.05 — — Comparative rail steel 63 1.19 0.10 0.41 —— 0.09 — — — 64 0.77 0.51 1.36 — — — — — — 65 0.78 0.54 1.30 — — — — — —66 0.82 0.25 1.05 0.25 — — — — — 67 0.81 0.28 1.08 0.21 — — — — — 680.82 0.49 1.10 0.22 — — — — — 69 0.82 0.51 1.12 0.24 — — — — —

TABLE 8 accelerated maximum wear existence of the cooling rate hardnessof hardness of amount of occurrence of the of gage gage corner head togage comer surface damage at comet portion portion portion portion thehead top portion rail No. (° C./sec) (HV) (HV) (mm) (1,000,000 times)Present 54 3 385 288 1.8 no damage occurrence rail steel 55 10  392 2751.9 ″ 56 3 402 305 1.7 ″ 57 1 411 300 1.6 ″ 58 5 384 285 1.3 ″ 59 3 398294 1.2 ″ 60 2 380 271 1.2 ″ 61 3 384 292 1.2 ″ 62 3 416 304 0.8 ″ 63 2421 315 0.6 ″ Comparative 64  4* 392 388 3.7 damage occurred rail steel65 4 388 305 3.8 no damage occurrence 66  3* 396 390 3.4 damage occurred67 3 391 319 3.5 no damage occurrence 68  3* 405 399 3.1 damage occurred69 3 400 315 3.2 no damage occurrence *Accelerated cooling was appliedto the head top portion at the same cooling rate as the gage cornerportion.

Further, Table 8 also represents the maximum wear amount of the gagecorner portion of the rail testpiece by a water lubrication rollingfatigue tester using disc testpieces 6 and 7 reduced to ¼ the exact sizeof the rail and the wheel shape shown in FIG. 10 and the existence ofthe occurrence of the surface damage at the head top portion. FIG. 11comparatively shows the maximum wear quantity of the gage cornerportions of the present rail steels and the Comparative rail steels.

By the way, the construction of the rails is as follows.

Present rails (10 rails) Nos. 54 to 63

Heat-treated rails having a hardness of not less than Hv 360 at the gagecorner portion and a hardness of Hv 250 to 320 at the head top portion,having the components within the range described above, and applied withaccelerated cooling at the gage corner portion thereof.

Comparative rails (6 rails) Nos. 64 to 69

-   -   Comparative rails by eutectoid carbon-containing steel.    -   The condition of the rolling fatigue test is as follows.    -   Testing machine: rolling fatigue tester (see FIG. 10)    -   Shape of testpiece: disc-like testpiece (outer diameter=200 mm,        sectional shape of rail material, ¼ model of 136 pound-rail)    -   Test load:        -   radial load: 2.0 tons        -   thrust load: 0.5 tons    -   Angle of torsion: 0.5° (reproduction of sharp curve)    -   Atmosphere: dry+water lubrication (60 cc/min)    -   Number of revolution: dry: 100 rpm, water lubrication: 300 rpm)    -   Number of times of repetition:        -   Dry state to 5,000 times, and thereafter test was conducted            to 700,000 times with water lubrication.

As tabulated in Table 7, the present rail steels increase the carboncontent in comparison with the Comparative rail steels and at the sametime, provide the difference of the hardness in the hardnessdistribution of the section by the heat-treatment so that the hardnessof the gage corner portion is higher than that of the head top portionas shown in FIG. 9. Accordingly, the maximum wear amount of the gagecorner portion is smaller than that of the Comparative rails, and thesurface damage resistance at the head top portion is equal to theComparative rails in which the hardness of the gage corner portion ishigher than that of the head top portion.

Example 5

This Example relates to the improvement of the weld joint portion. Table9 tabulates the principal chemical components of the present rail steelof this Example and a Comparative rail steel.

TABLE 9 principal chemical composition (wt %) Si + Cr + Mn C Si Nn Cr(wt %) present 0.90 0.88 0.60 0.58 2.06 rail steel Comparative 0.91 0.46058 0.21 1.25 rail steel

Incidentally, the construction of each rail is as follows.

Present rail steel

Heat-treated rail having the components listed above, and a pearlitelamella space of not greater than 100 nm. Accelerated cooling wasapplied to the head portion having a ratio of the cementite thickness tothe ferrite thickness of at least 0.15 in the pearlite structure.

Comparative rail steel

A Comparative steel by an eutectoid carbon-containing steel.

The flash butt welding condition is as follows.

-   -   Welding machine: Model K-355    -   Capacity: 150 KVA    -   Secondary current: 20,000 amp, maximum    -   Clamp force: 125 t, maximum    -   Upset amount: 10 mm

FIG. 12 shows the hardness values of the steels of this Example afterwelding by the relation between the hardness and the distance from aweld line. It can be appreciated from this diagram that in the railsteel according to the present invention, the drop of the hardness onthe weld line due to decarburization can be improved, and the drop ofthe hardness clue to sphering of the heat affected portion tends todecrease. Further, the difference of the hardness from the hardness ofthe base metal is not greater than 30 in terms of Hv at weld portionsother than at the position where an extreme drop of the hardness occurs.

INDUSTRIAL APPLICABILITY

The rail steels according to the present invention increase the carboncontent to a higher content than the conventional rail steels, narrowthe lamella space in the pearlite structure, further restrict thecementite thickness to the ferrite thickness so as to improve breakageresistance due to machining of the pearlite, and obtain the high wearresistance and the high damage resistance by reducing the hardness ofthe weld portion. Further, the present invention makes it possible toshorten the heat-treatment process and to improve producibility.

1. A pearlitic steel rail, having a good wear resistance, comprisingmore than 0.85 to 1.20%, in terms of percent by weight, of carbon,characterized in that the structure of said steel rail is a pearlite, apearlite lamella space of said pearlite is not more than 100 nm, and aratio of a cementite thickness to a ferrite thickness in said pearliteis at least 0.15.
 2. A pearlitic steel rail, having a good wearresistance, comprising more than 0.85 to 120%, in terms of percent byweight, of carbon, characterized in that the structure within the rangeof a depth of 20 mm from the surface of a rail head portion of saidsteel rail with said head surface being the start point is pearlite, apearlite lamella space of said pearlite is not more than 100 nm, and aratio of a cementite thickness to a ferrite thickness in said pearliteis at least 0.15.
 3. A pearlite type steel rail, having a good wearresistance, comprising, in terms of percent by weight: C: more than 0.85to 1.20%, Si: 0.10 to 1.00%, Mn: 0.40 to 1.50%, and the balanceconsisting of iron and unavoidable impurities, said steel railcharacterized in that the structure of said steel rail is pearlite, apearlite lamella space of said pearlite is not more than 100 nm, and aratio of a cementite thickness to a ferrite thickness in said pearliteis at least 0.15.
 4. A pearlitic steel rail having a good wearresistance, comprising, in terms of percent by weight: C: more than 0.85to 1.20%, Si: 0.10 to 1.00%, Mn: 0.40 to 1.50%, and the balanceconsisting of iron and unavoidable impurities, said steel railcharacterized in that the structure within the range of a depth of 20 mmfrom the surface of a rail head portion of said steel rail with saidhead surface being the start point is pearlite, a pearlite lamella spaceof said pearlite is not more than 100 nm, and a ratio of a cementitethickness to a ferrite thickness in said pearlite is at least 0.15.
 5. Apearlitic steel rail having a good wear resistance, comprising, in termsof percent by weight: C: more than 0.85 to 1.20%, Si: 0.10 to 1.00%, Mn:0.40 to 1.50, at least one member selected from the group consisting of:Cr: 0.05 to 0.50%, Mo: 0.01 to 0.20%, V: 0.02 to 0.30%, Nb: 0.002 to0.05%, Co: 0.10 to 2.00%, B: 0.0005 to 0.005%, and the balanceconsisting of iron and unavoidable impurities, said steel railcharacterized in that the structure of said steel rail is pearlite, apearlite lamella space in said pearlite is not more than 100 nm, and aratio of a cementite thickness to a ferrite thickness in said pearlitestructure is at least 0.15.
 6. A pearlitic steel rail having a good wearresistance, comprising, in terms of percent by weight: C: more than 0.85to 1.20%, Si: 0.10 to 1.00%, Mn: 0.40 to 1.50%, at least one memberselected from the group consisting of: Cr: 0.05 to 0.50%, Mo: 0.01 to0.20%, V: 0.02 to 030%, Nb: 0.002 to 0.05%, Co: 0.10 to 2.00%, B: 0.0005to 0.005%, and the balance consisting of iron and unavoidableimpurities, said steel rail characterized in that the structure withinthe range of a depth of 20 mm from the surface of a rail head portion ofsaid steel rail with said head surface being the start point ispearlite, a pearlite lamella space of said pearlite is not more than 100nm, and a ratio of a cementite thickness to a ferrite thickness in saidpearlite is at least 0.15.
 7. A pearlitic steel rail having a goodweldability and a high wear resistance according to claim 1, wherein thedifference of hardness between a weld joint portion and a base metal isnot more than Hv
 30. 8. A pearlite type steel rail having a goodweldability and a good wear resistance according to claim 3 wherein saidchemical components Si, Cr and Mn satisfy the relation Si+Cr+Mn=1.5 to3.0% in terms of percent by weight.
 9. A method for producing apearlitic steel rail as defined in any of claims 1 to 6, said methodcomprising the steps of: hot rolling a melted and cast steel to providea steel rail, with said steel rail retaining rolling heat immediatelyafter hot rolling; cooling in an accelerated manner said steel railretaining rolling heat immediately after hot rolling or cooling in anaccelerated manner said steel rail heated for heat treatment, saidaccelerated cooling taking place from an austenite temperature at acooling rate of 1° to 10° C./sec; stopping said accelerated cooling atthe point when said steel rail temperature reaches 700° to 500° C.; andthereafter leaving said steel rail to cool; wherein the hardness of saidsteel rail within the range of a depth of 20 mm from the surface of ahead portion of said steel rail is at least Hv
 320. 10. A method forproducing a pearlitic steel rail as defined in any of claims 1 to 6having a good wear resistance, said method comprising the steps of: hotrolling a melted and cast steel to provide a steel rail, with said steelrail retaining rolling heat immediately after hot rolling; cooling in anaccelerated manner said steel rail retaining rolling heat immediatelyafter hot rolling or cooling in an accelerated manner said steel railheated for heat treatment, said accelerated cooling taking place from anaustenite temperature at a cooling rate of more than 10° C./sec and upto 30° C./sec; stopping said accelerated cooling at the point whenpearlite transformation of said steel rail has proceeded at least 70%and the temperature of the rail is 700 to 500° C.; and thereafterrecuperating heat from within the rail while leaving said steel rail tocool; wherein the hardness of said steel rail within the range of adepth of 20 mm from the surface of a head portion of said steel rail isat least Hv 320, wherein said steel rail comprises 0.86 to 1.20%, interms of percent by weight of carbon, wherein the structure of saidsteel rail is a pearlite, a pearlite lamella space of said pearlite isnot more than 100 nm, and a ratio of a cementite thickness to a ferritethickness in said pearlite is at least 0.15, and wherein said steel railis capable of being used as part of a heavy load railway for trainshaving a wheel weight of 15 tons.
 11. A method for producing a pearliticsteel rail as defined in any of claims 1 to 6, said method comprisingthe steps of: hot rolling a melted and cast steel to provide a steelrail, with said steel rail retaining rolling heat immediately after hotrolling; cooling in an accelerated manner said steel rail retainingrolling heat immediately after hot rolling or cooling in an acceleratedmanner said steel rail heated for heat treatment, said acceleratedcooling taking place from an austenite temperature at a cooling rate of1° to 10° C./sec: stopping said accelerated cooling at the point whenthe temperature of a gage corner portion of said steel rail reaches 700°to 500° C.; and thereafter leaving said steel rail to cool; wherein thehardness of said gage corner portion of said steel rail is at least Hv360 and the harness of a head top portion is Hv 250 to
 320. 12. A methodfor producing a pearlitic steel rail as defined in any of claims 1 to 6,said method comprising the steps of: hot rolling a melted and cast steelto provide a steel rail, with said steel rail retaining rolling heatimmediately after hot rolling; cooling in an accelerated manner saidsteel rail retaining rolling heat immediately after hot rolling orcooling in an accelerated manner said steel rail heated for heattreatment, said accelerated cooling taking place from an austenitetemperature at a cooling rate of more than 10° C./sec and up to 30°C./sec.; stopping said accelerated cooling at the point when pearlitetransformation of a gage corner portion of said steel rail has proceededat least 70%; and thereafter leaving said steel rail to cool; whereinthe hardness of said gage corner portion of said steel rail is at leastHv 360 and the hardness of a head top portion is Hv 250 to
 320. 13. Amethod for producing a pearlitic steel rail as defined in claim 8, saidmethod comprising the steps of: hot rolling a melted and cast steel toprovide a steel rail, with said steel rail retaining rolling heatimmediately after hot rolling; cooling in an accelerated manner saidsteel rail retaining rolling heat immediately after hot rolling orcooling in an accelerated manner said steel rail heated for heattreatment, said accelerated cooling taking place from an austenitetemperature at a cooling rate of 1° to 10° C./sec.; stopping saidaccelerated cooling at the point when the temperature of said railreaches 700° to 500° C.; and thereafter leaving said steel rail to cool;wherein the hardness within the range of a depth of 20 mm from thesurface of a head portion of said steel rail is at least Hv
 320. 14. Themethod for producing a pearlitic steel rail having a good weldabilityand good wear resistance according to claim 10, wherein said steel railcomprises a weld joint portion formed by welding a joint portion, andwherein the difference of hardness between the weld joint portion andbase metal in the joint portion prior to welding is not more than Hv 30.15. The method for producing a pearlitic steel rail having a goodweldability and good wear resistance according to claim 10, wherein themethod further comprises welding a joint portion to form a weld jointportion.
 16. The method for producing a pearlitic steel rail having agood weldability and good wear resistance according to claim 15, whereinthe weld joint portion has undergone flash butt welding.
 17. The methodfor producing a pearlitic steel rail having a good weldability and goodwear resistance according to claim 10, wherein a hardness of a gagecorner portion is higher than a hardness of a head top portion.
 18. Amethod for producing a pearlitic steel rail, having a good wearresistance, said method comprising the steps of: hot rolling a meltedand cast steel to provide a steel rail, with said steel rail retainingrolling heat immediately after hot rolling; cooling in an acceleratedmanner said steel rail retaining rolling heat immediately after hotrolling, said accelerated cooling taking place from an austenitetemperature at a cooling rate of more than 10° C./sec and up to 30°C./sec; stopping said accelerated cooling at the point when pearlitetransformation of said steel rail has proceeded at least 70% and thetemperature of the rail is 700 to 500° C.; and thereafter recuperatingheat from within the rail while leaving said steel rail to cool, whereinthe hardness of said steel rail within the range of a depth of 20 mmfrom the surface of a head portion of said steel rail is at least Hv320, wherein said steel rail comprises 0.86 to 1.20%, in terms ofpercent by weight of carbon, characterized in that the structure withinthe range of a depth of 20 mm from the surface of a rail head portion ofsaid steel rail with said head surface being the start point ispearlite, a pearlite lamella space of said pearlite is not more than 100nm, and a ratio of a cementite thickness to a ferrite thickness in saidpearlite is at least 0.15, and wherein said steel rail is capable ofbeing used as part of a heavy load railway for trains having a wheelweight of 15 tons.
 19. The method for producing a pearlitic steel railhaving a good weldability and good wear resistance according to claim18, wherein said steel rail comprises a weld joint portion formed bywelding a joint portion, and wherein the difference of hardness betweenthe weld joint portion and base metal in the joint portion prior towelding is not more than Hv
 30. 20. The method for producing a pearliticsteel rail having a good weldability and good wear resistance accordingto claim 18, wherein the method further comprises welding a jointportion to form a weld joint portion.
 21. The method for producing apearlitic steel rail having a good weldability and good wear resistanceaccording to claim 20, wherein the weld joint portion has undergoneflash butt welding.
 22. A method for producing a pearlitic steel rail,having a good wear resistance said method comprising the steps of: hotrolling a melted and cast steel to provide a steel rail, with said steelrail retaining rolling heat immediately after hot rolling; cooling in anaccelerated manner said steel rail retaining rolling heat immediatelyafter hot rolling, said accelerated cooling taking place from anaustenite temperature at a cooling rate of more than 10° C./sec and upto 30° C./sec; stopping said accelerated cooling at the point whenpearlite transformation of said steel rail has proceeded at least 70%and the temperature of the rail is 700 to 500° C.; and thereafterrecuperating heat from within the rail while leaving said steel rail tocool, wherein the hardness of said steel rail within the range of adepth of 20 mm from the surface of a head portion of said steel rail isat least Hv 320, wherein said steel rail comprises, in terms of percentby weight: C: 0.86 to 1.20%, Si: 0.10 to 1.00%, Mn: 0.40 to 1.50%, andthe balance consisting of iron and unavoidable impurities, wherein saidsteel rail characterized in that the structure of said steel rail ispearlite, a pearlite lamella space of said pearlite is not more than 100nm, and a ratio of a cementite thickness to a ferrite thickness in saidpearlite is at least 0.15, and wherein said steel rail is capable ofbeing used as part of a heavy load railway for trains having a wheelweight of 15 tons.
 23. The method for producing a pearlitic steel railhaving a good weldability and good wear resistance according to claim22, wherein a hardness of a gage corner portion is higher than ahardness of a head top portion.
 24. A method for producing a pearliticsteel rail, having a good wear resistance, said method comprising thesteps of: hot rolling a melted and cast steel to provide a steel rail,with said steel rail retaining rolling heat immediately after hotrolling; cooling in an accelerated manner said steel rail retainingrolling heat immediately after hot rolling, said accelerated coolingtaking place from an austenite temperature at a cooling rate of morethan 10° C./sec and up to 30° C./sec; stopping said accelerated coolingat the point when pearlite transformation of said steel rail hasproceeded at least 70% and the temperature of the rail is 700 to 500°C.; and thereafter recuperating heat from within the rail while leavingsaid steel rail to cool, wherein the hardness of said steel rail withinthe range of a depth of 20 mm from the surface of a head portion of saidsteel rail is at least Hv 320, wherein said steel rail comprises, interms of percent by weight: C: 0.86 to 1.20%, Si: 0.10 to 1.00%, Mn:0.40 to 1.50%, and the balance consisting of iron and unavoidableimpurities, wherein said steel rail characterized in that the structurewithin the range of a depth of 20 mm from the surface of a rail headportion of said steel rail with said head surface being the start pointis pearlite, a pearlite lamella space of said pearlite is not more than100 nm, and a ratio of a cementite thickness to a ferrite thickness insaid pearlite is at least 0.15, and wherein said steel rail is capableof being used as part of a heavy load railway for trains having a wheelweight of 15 tons.
 25. A method for producing a pearlitic steel rail,having a good wear resistance, said method comprising the steps of: hotrolling a melted and cast steel to provide a steel rail, with said steelrail retaining rolling heat immediately after hot rolling; cooling in anaccelerated manner said steel rail retaining rolling heat immediatelyafter hot rolling, said accelerated cooling taking place from anaustenite temperature at a cooling rate of more than 10° C./sec and upto 30° C./sec; stopping said accelerated cooling at the point whenpearlite transformation of said steel rail has proceeded at least 70%and the temperature of the rail is 700 to 500° C.; and thereafterrecuperating heat from within the rail while leaving said steel rail tocool, wherein the hardness of said steel rail within the range of adepth of 20 mm from the surface of a head portion of said steel rail isat least Hv 320, wherein said steel rail comprises, in terms of percentby weight: C: 0.86 to 1.20%; Si: 0.10 to 1.00%; Mn: 0.40 to 1.50%; atleast one member selected from the group consisting of: Cr: 0.05 to0.50%; Mo: 0.01 to 0.20%; V: 0.02 to 0.30%; Nb: 0.002 to 0.05%; Co: 0.10to 2.00%; B: 0.0005 to 0.005%; and the balance consisting of iron andunavoidable impurities, wherein said steel rail characterized in thatthe structure of said steel rail is pearlite, a pearlite lamella spacein said pearlite is not more than 100 nm, and a ratio of a cementitethickness to a ferrite thickness in said pearlite structure is at least0.15, and wherein said steel rail is capable of being used as part of aheavy load railway for trains having a wheel weight of 15 tons.
 26. Themethod for producing a pearlitic steel rail having a good weldabilityand good wear resistance according to claim 25, wherein a hardness of agage corner portion is higher than a hardness of a head top portion. 27.A method for producing a pearlitic steel rail, having a good wearresistance, said method comprising the steps of: hot rolling a meltedand cast steel to provide a steel rail, with said steel rail retainingrolling heat immediately after hot rolling; cooling in an acceleratedmanner said steel rail retaining rolling heat immediately after hotrolling, said accelerated cooling taking place from an austenitetemperature at a cooling rate of more than 10° C./sec and up to 30°C./sec; stopping said accelerated cooling at the point when pearlitetransformation of said steel rail has proceeded at least 70% and thetemperature of the rail is 700 to 500° C.; and thereafter recuperatingheat from within the rail while leaving said steel rail to cool, whereinthe hardness of said steel rail within the range of a depth of 20 mmfrom the surface of a head portion of said steel rail is at least Hv320, wherein said steel rail comprises, in terms of percent by weight:C: 0.86 to 1.20%; Si: 0.10 to 1.00%; Mn: 0.40 to 1.50%; at least onemember selected from the group consisting of: Cr: 0.05 to 0.50%; Mo:0.01 to 0.20%; V: 0.02 to 0.30%; Nb: 0.002 to 0.05%; Co: 0.10 to 2.00%;B: 0.0005 to 0.005%; and the balance consisting of iron and unavoidableimpurities, wherein said steel rail characterized in that the structurewithin the range of a depth of 20 mm from the surface of a rail headportion of said steel rail with said head surface being the start pointis pearlite, a pearlite lamella space of said pearlite is not more than100 nm, and a ratio of a cementite thickness to a ferrite thickness insaid pearlite is at least 0.15, and wherein said steel rail is capableof being used as part of a heavy load railway for trains having a wheelweight of 15 tons.